Process for producing sponge iron and reduced iron powder sponge iron and charging apparatus

ABSTRACT

A method for manufacturing sponge iron and an apparatus for charging in the method are disclosed. Iron oxide powder and reducing-agent powder are charged such that alternating layers of the iron oxide powder and the reducing-agent powder are formed and such that each of the layers is in the form of a helix, and then a reduction treatment is performed. The method has not only high reaction efficiency of a gas, high quality, and high productivity, but also the advantage for a production adjustment because the amount of charge can be adjusted without the limitation of a reduction time. The molar ratio of the carbon content in the reducing agent to the oxygen content in the iron oxide in the reaction container is preferably 1.1 or more.

TECHNICAL FIELD

The present invention relates to a method for manufacturing sponge ironused as a material in manufacturing iron powder and a method formanufacturing reduced iron powder with the sponge iron manufactured bythe method.

The reduced iron powder is used in the form of powder as-is, and alsoused as a material for a sintered product such as a mechanical componentand a magnetic material.

The present invention also relates to an apparatus for charging amaterial of sponge iron manufactured by the method for manufacturing asponge iron, and relates to high-purity sponge iron that can bemanufactured by the method.

BACKGROUND ART

FIGS. 1A and 1B shows a general process for manufacturing sponge iron.FIG. 1A is a vertical sectional view illustrating a state of materialscharged in a container. FIG. 1B is a horizontal sectional viewillustrating a state of materials charged in a container.

Sponge iron is manufactured by the following procedure: Iron oxidepowder 2 and reducing-agent powder 3 are alternately charged in the formof coaxial cylinders into a cylindrical heat-resistant reactioncontainer 1 (sagger) that can be equipped with a lid at the bottom. Thecharged iron oxide powder 2 and reducing-agent powder 3 are heated(indirectly heated) at 1050° C. to 1200° C. in the reaction container 1with a tunnel furnace or the like. The iron oxide powder 2 in thereaction container 1 is reduced (roughly reduced) and is sintered by theheat treatment, thus resulting in metallic iron that is in the form of asponge, i.e., sponge iron.

The iron oxide powder 2 includes iron ore powder and powder produced bycrushing mill scale. The reducing-agent powder 3 includes coke powderand coal powder. Lime powder or the like may be added to thereducing-agent powder 3, if necessary.

The above-described techniques are disclosed in “Tekkou binran”, thirdedition, vol. 5, pp. 457-459 (in particular, page 457, right column,line 10-13) and Japanese Unexamined Patent Application Publication No.2002-241822.

In a known technique for manufacturing sponge iron as shown in FIGS. 1Aand 1B, the iron oxide powder 2 is cylindrically charged into thereaction container 1 (hereinafter, referred to as “cylindricaliron-oxide layer”). The reducing-agent powder 3 surrounds thiscylindrical iron-oxide layer and is charged into above, below, andinside of the cylindrical iron-oxide layer.

When the reaction container 1 is heated after the materials are charged,in an early stage, a carbon dioxide (CO₂) gas formed by allowing oxygenthat is present in the voids of the charged layer of the reducing agentto react with carbon in the reducing agent and formed by thedecomposition of limestone added to the reducing agent reacts withcarbon in the reducing agent according to chemical equation (1) togenerate carbon monoxide (CO), which is a reducing gas, in the chargedlayer of the reducing-agent powder 3 (reducing-agent layer).C+CO₂→2CO  equation (1)

The CO gas thus generated reaches from the reducing-agent layer to acharged layer of the iron oxide powder 2 (iron-oxide layer). Then, ironoxide is reduced as the generation of a CO₂ gas according to thefollowing chemical equation (2):FeOn+nCO→Fe+nCO₂  equation (2)

The generated CO₂ gas diffuses into the iron-oxide layer includingpartially-reduced iron oxide and reaches the reducing-agent layer again.Then the CO₂ gas reacts with carbon in the reducing-agent layer togenerate a CO gas according to equation (1). This resulting CO gasdiffuses into the iron-oxide layer again and reacts with unreduced ironoxide according to equation (2) to produce iron as the generation of aCO₂ gas.

As a result, all iron oxide powder 2 charged in the reaction container 1is reduced to iron powder by repeating the reactions according toequations (1) and (2) at certain intervals. At the same time of thisreduction reaction, reduced iron particles are sintered to formcylindrical sponge iron (sintered body). FIG. 2 shows an appearance of asponge iron produced by a known art (lower part is omitted).

An amount of CO gas required for reducing all iron oxide istheoretically 1 in molar ratio according to equation (2) ((the number ofmoles of carbon atoms in the CO gas)/(the number of moles of oxygenatoms in the iron oxide)). Hence, an amount of reducing agent requiredfor reducing all iron oxide is 1.0 in molar ratio ((the number of molesof carbon atoms in the reducing agent)/(the number of moles of oxygenatoms in the iron oxide)). Hereinafter, (the number of moles of carbonatoms in the reducing agent)/(the number of moles of oxygen atoms in theiron oxide) is referred to as “(the carbon content)/(the oxygen content)(molar ratio)”.

DISCLOSURE OF INVENTION

In the above-described process for reducing, diffusion of the CO and CO₂gases which are generated in the reaction container 1 into the ironoxide powder 2 and reducing-agent powder 3 is a main rate-determiningfactor in the reduction reaction. However, in a process having thestructure charged as shown in FIG. 1, there is a problem in that ittakes a long time required for the reduction because of the longdiffusion lengths of the CO and CO₂ gases.

For example, in a manufacturing step for an industrial-scale productionwith a tunnel furnace for heating, a long time required for thereduction decreases reaction efficiency (gas use efficiency); hence, ittakes several days from charging materials to drawing a product, thusleading to low productivity. Furthermore, heating energy consumptionrequired for the reduction is significantly large.

In a process having a charged structure as shown in FIGS. 1A and 1B,although it is necessary to increase a thickness (radial direction) ofthe layer of the iron oxide powder 2 in order to increase the yield ofsponge iron manufactured, in this case, long reduction time is required.When the thickness of the layer of the iron oxide powder 2 is reduced inorder to shorten the reduction time, an amount of sponge iron that canbe manufactured per reaction container is decreased. Hence, it does notnecessarily lead to the improvement of the yield per unit time.

Therefore, a combination of the thickness of the layer of the iron oxidepowder 2 and the reduction time is uniquely determined so that thelargest yield can be achieved. There are problems with a low degree offlexibility in adjusting the yield as well as the limitation of theyield.

In addition, in a process for charging as shown in FIGS. 1A and 1B, a COgas generated by the above-described reaction tends to flow through thelayer, which has a lower density, of the reducing-agent powder 3 andthen go out of the reaction container 1. Consequently, the CO gas doesnot effectively contribute to the reduction reaction.

Furthermore, to hold shape of the layer of the iron oxidepowder 2 in afiring stage, it is necessary to excessively charge the reducing-agentpowder 3 into a portion between the reaction container 1 and the ironoxide powder 2 and into inside of the cylindrical iron-oxide layer.

In the above-described circumstances in a known process, there is aproblem in that a large amount of reducing-agent powder 3 is required,i.e., at least 2.0 in molar ratio, thus resulting in poor unitrequirement of a reducing agent.

In addition, the lower portion of a cylindrical iron-oxide layer canswell under its own weight. Hence, there is a problem in that iron oxidein the swelling portion is insufficiently reduced within a predeterminedreduction time, thus remaining an unreduced portion.

It is an object of the present invention to advantageously solve variousproblems described above of the known art. That is, it is an object ofthe present invention to provide a method for manufacturing sponge iron,wherein the method has high productivity and can easily adjust theyield.

It is another object of the present invention to provide an apparatusfor charging materials into a reaction container, wherein the apparatusis advantageously used when the above-described method for manufacturingis performed.

The inventors have conducted intensive research, and found that theabove-described problems can be advantageously solved by devising acharged form of iron oxide powder and reducing-agent powder in areaction container. Consequently, the present invention has beencompleted.

That is, a first aspect of the present invention, a method formanufacturing sponge iron includes a charging step of charging ironoxide powder and reducing-agent powder into a reaction container; and areducing step of reducing the iron oxide powder in the reactioncontainer to produce a mass of sponge iron by heating from the outsideof the reaction container, wherein, in the charging step, the iron oxidepowder and the reducing-agent powder are charged such that alternatinglayers of the iron oxide powder and the reducing-agent powder are formedand such that each of the layers is in the form of a helix.

In the above-described first aspect of the present invention, suitableconditions described below are preferably applied alone or in anycombination.

(1) In the charging step, the iron oxide powder and the reducing-agentpowder are charged such that layers composed of the reducing-agentpowder are disposed on an inner side-surface of the reaction container(referred to as “peripheral portion”) and disposed at a central portionalong the vertical central axis and such that the alternating layersthat are in the form of helices are disposed at a portion (referred toas “intermediate portion”) other than the portion of the layers disposedon the inner side-surface and at the central portion. The peripheralportion and the central portion along the vertical central axiscorrespond to a circumferential portion and a central portion,respectively, in horizontal sectional view of the container. Theintermediate portion is preferably in the form of a cylinder or acolumn. When the reaction container is in the form of cylinder, thevertical central axis corresponds to the center of the cylinder.

(2) The iron oxide powder is composed of at least one selected from thegroup consisting of an iron ore, mill scale, and iron oxide powderrecovered from waste pickle liquor.

(3) The reducing-agent powder is composed of at least one selected fromthe group consisting of coke, char, and coal.

(4) A source of a carbon dioxide gas is added to the reducing-agentpowder. The source of a carbon dioxide gas preferably includes limestone(including calcined limestone). In this case, the reducing-agent powderto which the powder of the source of a carbon dioxide gas is added ischarged.

(5) The heating temperature is 1000° C. to 1300° C. in the reducingstep.

(6) In the charging step, the thicknesses of the layers of the ironoxide powder and the reducing-agent powder are variable when forming thelayers that are in the form of helices. Variably controlling includesthe following meanings: A different thickness of at least any one of thelayers can be set in each reaction container. A thickness of at leastany one of the layers can be varied with position of the reactioncontainer 1.

(7) In the charging step, the amounts of iron oxide powder andreducing-agent powder in the reaction container are controlled such thatthe molar ratio of the carbon content in the reducing-agent powder tothe oxygen content in the iron oxide powder is at least 1.1. The molarratio is preferably 1.15 or more and more preferably 1.2 or more.

(8) In the charging step according to suitable conditions (1) and (7),the amounts of iron oxide powder and reducing-agent powder in thecharged portion having layered structure are controlled such that themolar ratio of the carbon content in the reducing-agent powder to theoxygen content in the iron oxide powder is at least 0.5. The term“charged portion having layered structure” represents a cylindricalregion formed of helically deposited layers of iron oxide powder andreducing-agent powder. The region usually corresponds to a portion otherthan “layers composed of the reducing-agent powder” described in (1).

A second aspect of the present invention is a method for manufacturingreduced iron powder, the method including the steps of pulverizingsponge iron manufactured by the method according to the first aspect;reducing the resulting pulverized iron; and repulverizing the resultingreduced iron.

Suitable conditions (1) to (8) in the first aspect of the presentinvention can be applied in any combination.

A third aspect of the present invention is sintered sponge iron having ahelical form. The sponge iron preferably has high purity, i.e., has ametallic iron content of at least 97 percent by mass. In the firstaspect of the present invention, even a mass of the high-purity spongeiron having a weight of 100 kg or more can be manufactured by, forexample, particularly applying suitable condition (7) and subjecting toreduction treatment for sufficient time.

A fourth aspect of the present invention is an apparatus for chargingmaterials used to manufacture sponge iron into a container, thematerials being iron oxide powder and reducing-agent powder, theapparatus including a charger capable of rotating and vertically movingin the container when the charger is disposed in the container; anoutlet for the iron oxide powder and an outlet for the reducing-agentpowder, these outlets being provided at the bottom of the charger andcapable of rotating together with the charger.

For the method for manufacturing sponge iron according to the firstaspect of the present invention, the fourth aspect of the presentinvention is preferably employed to charge the iron oxide powder and thereducing-agent powder such that alternating layers of the iron oxidepowder and the reducing-agent powder are disposed and such that each ofthe layers is in the form of a helix.

In the fourth aspect of the present invention, the opening areas of theoutlet for the iron oxide powder and the outlet for the reducing-agentpowder is preferably variable. Such a structure can be preferably usedto particularly satisfy suitable condition (6).

In the fourth aspect of the present invention, the charger preferablyincludes a cylindrical main body having a diameter of up to 85% of theinside diameter of the container; and a lower end composed of part of acylinder, the horizontal section of the cylinder being a circle having adiameter of 90% to 95% of the inside diameter of the container, whereinthe horizontal section of the lower end has the shape of a sectorincluding the center of the circle and part of the circumference of thecircle, or has a shape including the sector. Such a structure can bepreferably used to reduce the thickness of the layer composed of thereducing-agent powder disposed at the peripheral portion described insuitable condition (1). Furthermore, even when a projection composed ofan adherent is produced in the reaction container, the above-describedcharger can be disposed without interference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view illustrating a known process forcharging iron oxide powder and reducing-agent powder;

FIG. 1B is a horizontal sectional view taken along line IB-IB′ in FIG.1A;

FIG. 2 is a perspective view showing an appearance of sponge ironproduced by a known process;

FIG. 3A is a cross-sectional view illustrating an example of a methodfor charging iron oxide powder and reducing-agent powder according tothe present invention;

FIG. 3B is a horizontal sectional view taken along line IIIB-IIIB′ inFIG. 3A;

FIG. 4A is a schematic diagram showing an example of a structure of acharger (rotatable charging cylinder) of the present invention;

FIG. 4B is a cross-sectional view showing a charging state when usingthe rotatable charging cylinder;

FIG. 5 is a schematic diagram showing another example of a structure ofa charger (rotatable charging cylinder) of the present invention;

FIG. 6 is a cross-sectional view illustrating another example of amethod for charging iron oxide powder and reducing-agent powderaccording to the present invention;

FIG. 7 is a perspective view showing an appearance of sponge ironproduced by the present invention;

FIG. 8 is a cross-sectional view illustrating an experimental example ofa method for charging iron oxide powder and reducing-agent powder whichare in the form of horizontal multiple layers;

FIG. 9 is a graph showing the relationship between (the carboncontent)/(the oxygen content) (in molar ratio) (horizontal axis) in theentire reaction container and time required for reduction (verticalaxis) with reference to various thicknesses of iron oxide layers in amethod of alternating charging;

FIG. 10 is a cross-sectional view illustrating another experimentalexample of a method for charging iron oxide powder and reducing-agentpowder which are in the form of horizontal multiple layers;

FIG. 11 is a graph showing the relationship between (the carboncontent)/(the oxygen content) (in molar ratio) (horizontal axis) in theportion charged in the form of alternating layers and time required forreduction (vertical axis) with reference to various thicknesses of ironoxide layers in another method of alternating charging;

FIG. 12 is a graph showing the relationship between (the carboncontent)/(the oxygen content) (in molar ratio) (horizontal axis) in theentire reaction container and time required for reduction (verticalaxis) with reference to various thicknesses of iron oxide layers in theanother method of alternating deposition;

FIG. 13 is a graph showing the relationship between the increment ofiron oxide (percent by weight, horizontal axis) and the purity ofmetallic iron obtained by the reduction (percent by mass, vertical axis)with reference to charging in an interwound helical form (hatchingpatterned bars) and charging in a cylindrical form (outline bars);

FIG. 14A is a cross-sectional view showing yet another example of astructure of a charger (rotatable charging cylinder); and

FIG. 14B is an arrow view showing a cross-section taken along lineXIVB-XIVB′ in FIG. 14A (the thickness of the wall is omitted).

REFERENCE NUMERALS

1, 11 reaction container (sagger)

2, 12 iron oxide powder

3, 13 reducing-agent powder

14 apparatus for charging materials

14 a, 14 d partition wall

14 b rotatable charging cylinder

14 c cut-out section

15 outlet for iron oxide powder

16 outlet for reducing-agent powder (used for alternating charging)

16 a outlet for delivering reducing-agent powder to the peripheralportion

16 b outlet for delivering reducing-agent powder to the central axialportion

17 iron-oxide-powder holding section

18 reducing-agent-powder holding section

19 a, 19 b presser plate

a opening height

BEST MODE FOR CARRYING OUT THE INVENTION

[Method and Apparatus for Charging Materials]

The present invention is characterized by a method for chargingmaterials. The materials are iron oxide powder and reducing-agentpowder. Limestone and the like may be added to the reducing agent, ifnecessary.

As shown in FIG. 1, for example, a process for charging iron oxidepowder2 and reducing-agent powder 3, which are in the form of coaxialcylinders along the axial direction, into an upright heat-resistantreaction container 1 having a cylindrical shape is generally applied.Alternatively, the present invention employs a method for charging ironoxide powder and reducing-agent powder in helical forms. That is, ironoxide powder and reducing-agent powder are charged such that a helicallayer composed of the iron oxide powder and a helical layer composed ofthe reducing-agent powder are alternately stacked (hereinafter, referredto as “interwound helical charging”).

By employing a method for interwound helical charging, iron oxide powderand reducing-agent powder can be charged simultaneously andcontinuously. Therefore, a constant thickness of each layer (the amountof charged powder) can be obtained. Consequently, the thickness ratio ofa reducing agent layer to an iron oxide layer can also be maintainedconstant. This thickness ratio can be set to a desired ratio in eachreaction container depending on a purpose and circumstances.

In addition, the thickness ratio can also be changed to a desired valueat any time.

Consequently, the method for interwound helical charging is useful as amethod that is conducive to the improvement of productivity and yield.

FIGS. 3A and 3B show an example of the present invention. In chargingmaterials according to the present invention, it is preferable tosimultaneously charge iron oxide powder 12 and reducing-agent powder 13into a cylindrical reaction container 11 (sagger) composed of aheat-resistant material such as silicon carbide (SiC) with an apparatusfor charging materials 14.

The apparatus for charging materials 14 preferably has a structuredescribed below.

The apparatus for charging materials 14 mainly consists of a rotatablecharging cylinder 14 b (charger) that is inserted into the reactioncontainer 11. The cylindrical main body of the rotatable chargingcylinder 14 b is separated by a partition wall 14 a into twocompartments. The iron oxide powder 12 and the reducing-agent powder 13are charged into the two compartments, i.e., an iron-oxide-powderholding section 17 and a reducing-agent-powder holding section 18,respectively (each of the material powder is not shown in thecorresponding holding section). Furthermore, an outlet for iron oxidepowder 15 and an outlet for reducing-agent powder 16 are provided asopenings of the holding sections 17 and 18, respectively, at the lowerend (bottom or the neighborhood of the bottom) of the rotatable chargingcylinder 14 b. The degree of opening of each outlet (for example,opening height a), that is, the opening area can be preferably adjustedby a gate such as a sliding gate (not shown). The position and thedirection of each outlet may be determined according to need. Theopenings can be provided at any face selected from among theundersurface, the side face, and on a cut-out section provided at theundersurface of the rotatable charging cylinder 14 b. Each of thematerial powder charged into the corresponding holding section ispreferably delivered by its own weight in principle.

FIG. 4A is a detail view showing an example of the rotatable chargingcylinder 14 b. In this example, a cut-out section 14 c that is in theform of a square cylinder is disposed at a position extending from thecylinder bottom in a direction perpendicular to the partition wall 14 a.The two outlets (openings) 15 and 16 which are connected to the holdingsection 17 and 18 are provided at side walls that are diagonallyopposite each other of the cut-out section 14 c. FIG. 4B is across-sectional view showing a state charged with such a rotatablecharging cylinder.

Modification of this structure includes a structure in which each of thecut-out sections for iron oxide powder and reducing-agent powder has theshape of a sector that is about one quarter of a circle in horizontalsection and that is diagonally opposite each other. In this case, atleast part of the outlet 15 and at least part of the outlet 16 arepreferably provided at side faces, which are corresponding to a straightline of the sector, in the same plane through the axis of the rotatablecharging cylinder 14 b (a state illustrated in a cross-sectional view ofFIG. 3A is obtained).

FIG. 5 is a detail view showing another example of the rotatablecharging cylinder 14 b.

To surely charge material powder up to the circumferential portion ofthe reaction container 11 under control, the rotatable charging cylinder14 b preferably has a diameter close to the inner diameter of thereaction container 11. However, the reaction container is repeatedlyused, and a plurality of cylinders may be stacked to form a reactioncontainer. Hence, for example, reduced iron and ash in a reducing agentcan adhere to inside of the reaction container to form a projection. Inaddition, the container can slightly incline by strain caused byrepeated use. Therefore, the lower end of the rotatable chargingcylinder 14 b having a diameter very close to the inner diameter of thereaction container 11 can come in contact with the reaction container11, thus causing damage.

The purpose of bringing the lower end of the rotatable charging cylinder14 b closer to the inner diameter of the reaction container 11 is thatopenings extending from near the center to near the circumference of thereaction container are used as the outlets. Hence, if the positions ofthe outlets are modified, the lower end of the rotatable chargingcylinder 14 b need not have the shape of the perfect circle inhorizontal section. A sector that is part of this circle (virtualcircle) or a shape including at least the sector is adequate for thelower end.

FIG. 5 is an example of a lower end having the shape of a sector. Theoutlet for iron oxide powder 15 and the outlet for reducing-agent powder16 are asymmetrically provided at the side faces (corresponding tostraight lines of the sector) of the cut-out section 14 c that isprovided in the same way as shown in FIG. 4. Although the undersurfaceof the cut-out section 14 c is open, each powder 12 and 13 is mainlydelivered from the side face because deposited powder functions as theundersurface. Reference numerals 19 a and 19 b represent presser plates.

A desired central angle of a sector may be used. The central angle ispreferably about 180° (i.e., semicircle) or less in achieving asatisfactorily compact lower end. More preferably, the maximal diameterof a horizontal section of a cut-out section is smaller than thediameter of the virtual circle.

The virtual circle of the lower end desirably has a diameter closer tothe inner diameter of the reaction container in view of productivity,and preferably has a diameter of about 90% or more of the inner diameterof the reaction container. On the other hand, the virtual circle of thelower end desirably has an adequately small diameter in view ofoperation, and preferably has a diameter of about 95% or less of theinner diameter of the reaction container.

The rotatable charging cylinder 14 b preferably has a diameter of about85% or less of the inner diameter of the reaction container. Leaving aclearance for horizontal displacement in the container is preferable inorder to avoid contact. From the viewpoint of ensuring the pathway ofmaterial powder charged, the main body of the rotatable chargingcylinder has a diameter of about 30% or more of the inner diameter ofthe reaction container.

Interwound helical charging with such an apparatus for chargingmaterials 14 is performed as follows: Opening areas (the degrees ofopenings) of the outlets 15 and 16 are adjusted. The rotatable chargingcylinder 14 b is then inserted into the reaction container 11 fromabove. By moving upward the rotatable charging cylinder 14 b at aconstant speed while rotating the rotatable charging cylinder 14 b (thatis, rotating the outlets 15 and 16), the materials are charged(alternating charging) via the outlets such that the ratio of thethickness of the layer of iron oxide powder and the thickness of thelayer of reducing-agent powder is a constant and such that the layersare wound with each other. In this way, alternating layers of the ironoxide powder 12 and the reducing-agent powder 13, which are in the formof helices, is provided in the reaction container 11.

Materials are fed into the holding sections 17 and 18 before charging orduring charging into a reaction container, according to need.

FIG. 6 shows another example of a method for charging according to thepresent invention. The apparatus for charging materials 14 is shownschematically.

As shown in FIG. 6, in charging material powder into a reactioncontainer, a region where interwound helical charging is performed maybe limited to a region other than the peripheral portion along the axialdirection of the reaction container 11. In addition, a region whereinterwound helical charging is performed may be limited to a regionother than the central axial portion along the axial direction of thereaction container 11. Furthermore, a region where interwound helicalcharging is performed may be limited to a region other than both theperipheral portion and the central axial portion along the axialdirection of the reaction container 1. In all cases, a region whereinterwound helical charging is performed is referred to as “cylindricalintermediate portion”. The peripheral portion and the central axialportion correspond to the circumferential portion and the center of thecontainer in horizontal section.

The reducing-agent layer at the peripheral portion can be necessarilyprovided from the viewpoint of preventing the interference between therotatable charging cylinder 14 b of the apparatus for charging materials14 and the reaction container 11 and preventing the seizure at thecontact regions between the reaction container and the iron oxidepowder. The reducing-agent layer at the central axial portion can beprovided for handling reasons when removing sponge iron from thecontainer. In such a case, since a layer composed of a reducing agentalone is provided at the peripheral portion or the central axialportion, paths of the reaction gases are formed; hence, the gasesdiffuse in the container readily and uniformly. As a result, the effectof improving the reaction rate can be expected. In addition, thereducing-agent layer provided at the peripheral portion can also preventa product from adhering to the wall of the container. Therefore, thesereducing-agent layers are preferably provided while optimizing theradial thickness of the layer in view of the yield of a reducing agentand the molar ratio of (the carbon content)/(the oxygen content) and thelike, if necessary.

In a cylindrical container, a layer provided at the peripheral portionpreferably has a radial thickness of about 2.5% to about 5% of the innerdiameter of the container. The layer provided at the central axialportion preferably has a diameter of about 250 mm or less.

For example, to provide a reducing-agent layer at the peripheralportion, an opening is provided at the side of the rotatable chargingcylinder 14 a, and then reducing-agent powder may be delivered to form alayer at the peripheral portion. Furthermore, to provide areducing-agent layer at the central axial portion, a central tube havingan opening at its bottom is further provided at a position where thepartition wall 14 c is provided, and then reducing-agent powder may bedelivered from the opening to form a layer at the central axial portion.

These openings may be connected to the outlet 16 for providing a helicallayer or may be isolated.

FIG. 14A shows an example of a rotatable charging cylinder that cancharge in a state as shown in FIG. 6. FIG. 14B is a schematicalcross-sectional view taken along line XIVB-XIVB′ in FIG. 14A (thethickness of the wall is omitted for the simplification). In thisexample, the outlet for reducing-agent powder 16 is provided at theundersurface of the rotatable charging cylinder 14 b in order to chargein the form of alternating layers, for example, to charge in the form ofinterwound helices. Furthermore, an opening is provided at the side faceof the lower end of the rotatable charging cylinder 14 b, thusconstituting an outlet for delivering reducing-agent powder into theperipheral portion 16 a. In addition, an outlet for deliveringreducing-agent powder into the central axial portion 16 b is provided atthe center of the undersurface of the rotatable charging cylinder 14 b.A portion of reducing-agent powder is guided by a partition wall 14 d.

As shown in FIG. 3A, the bottom layer is usually composed ofreducing-agent powder (and limestone and the like) alone. As a result,the lower end of the iron oxide layer can be surely reduced, and theseizure between the reaction container and the iron oxide layer ispreferably blocked. The top layer is preferably composed ofreducing-agent powder alone for the same reasons. These reducing-agentlayers can be formed by, for example, closing the outlet for iron oxidepowder 15 of the apparatus for charging materials 14 or stopping thesupply of iron oxide powder to the rotatable charging cylinder 14 b.

In the present invention, when interwound helical charging is performedwith the above-described apparatus, it is preferable to variably controlthe thicknesses of the iron oxide layer and reducing-agent layer. Thatis, the thickness of each layer is preferably maintained constant ineach reaction container. However, it is preferable to be able to adjustthe thickness, for example, to optimize the thickness depending on amaterial.

Such a change in thickness of each layer can be achieved by adjusting atleast any two selected from, for example, the rotation speed and therising speed of the rotatable charging cylinder 14 b and the degrees ofopenings of the outlets 15 and 16. In particular, the adjustment of thedegrees of openings of the outlets 15 and 16 by, for example, openingand closing gates is preferable because a stable operation can beachieved without the reductions of diffusibility and the yield and theextension of reduction time.

The thickness of each layer can be varied continuously ordiscontinuously in theory with the height of the upright reactioncontainer 11, for example, can be varied at the bottom, the middle, andthe upper portion of the reaction container 11. The present inventiondoes not exclude such an application. An example of an applicationincludes that the thickness of the iron oxide layer is increased at theupper portion where the reduction tends to readily proceed.

An iron oxide layer and a reducing-agent layer, which are provided inthe form of helices, preferably have a thickness of at least about 5 mm.The sum of the thicknesses of the iron oxide layer and thereducing-agent layer is preferably at least about 10 mm and morepreferably at least 40 mm. Excessively small thickness readily resultsin an abnormal layer structure because of the fluctuation of thethickness of each layer. The lower limit of the thickness of each layeris more preferably at least about 10 mm. The lower limit of the sum ofthe thicknesses of the layers is more preferably at least about 30 mm.

On the other hand, excessively large thickness increases a time requiredfor the reduction treatment and reduces the material-efficiency. Hence,each of the layers preferably has a thickness of about 100 mm or less.The sum of the thicknesses of the layers (one iron oxide layer and onereducing-agent layer) preferably is about 200 mm or less. The upperlimit of the thickness of each layer is more preferably about 80 mm. Theupper limit of the sum of the thicknesses of the layers is morepreferably about 150 mm.

The ratio between an iron oxide layer and a reducing-agent layer isusually expressed not by the thickness but by (the carbon content)/(theoxygen content) (molar ratio). A preferable ratio will be describedbelow.

The above-described apparatus for charging materials is an example. Thatis to say, in an apparatus for charging iron oxide powder andreducing-agent powder into a reaction container, the apparatuspreferably includes a charger capable of rotating and vertically moving;and an outlet for the iron oxide powder and an outlet for thereducing-agent powder, these outlets being provided at the charger andcapable of rotating together with the charger. The apparatus can chargethe iron oxide powder and the reducing-agent powder from the outlets inthe form of a double helix by putting the charger into the reactioncontainer and then moving the charger upward while rotating the charger.

The charger advantageously has, for example, a cylindrical shape, but isnot limited to this. The charger may have a tubular shape whosecross-section is in the form of, for example, a sector, a star, or amultilobal according to the shape of a reaction container. The holdingsections need not be provided by separating the inside of the chargerwith a partition wall. Any shape and position of each holding sectionmay be used. The iron-oxide-powder holding section and thereducing-agent-powder holding section need not have the same capacities.

A fixed or movable guide plate and/or a presser plate are preferablyprovided around the outlets 15 and 16 in order to guide material powderto the direction desired.

[Material Powder]

In a method for manufacturing sponge iron according to the presentinvention, materials charged into a reaction container include at leastiron oxide powder and reducing-agent powder. The iron oxide powderpreferably includes a powdered iron ore or powdered mill scale generatedin a hot-rolling step of steel. A pickling step of removing, forexample, oxides formed on the steel products with an acid such ashydrochloric acid results in a waste acid (pickle liquor). An iron oxidepowder obtained by roasting this pickle liquor is also preferable as thematerial. Such an iron oxide powder preferably has an average particlesize of about 0.05 to about 10 mm.

Furthermore, finer iron oxide powder having a particle size smaller thanthat of the above-described iron oxide powder, for example, hematitepowder that is industrially controlled so as to have a specific surfacearea of at least 2 m²/g and a particle size of at least 0.01 μm is addedto the mill scale and/or the iron ore to produce a mixture. Theresulting mixture is preferably used for the material because themixture improves the quality of sponge iron.

Reducing-agent powder includes so-called carbonaceous powder containingcarbon. The carbonaceous powder preferably includes, for example, cokepowder, char (a kind of high-volatile charcoal), coal powder (noncakingcoal is preferable), anthracite powder, and charcoal. From the viewpointof the efficient reduction, the carbonaceous powder preferably has acarbon content of 60% or more. Reducing-agent powder preferably has anaverage particle size of about 0.05 to about 10 mm.

There is no problem in that reducing-agent powder containing powder thatis a source of a carbon dioxide gas is used as a material forreducing-agent layers, according to need. The source of a carbon dioxidegas preferably includes limestone (including hydrated lime).

[Reducing Step]

The iron oxide powder 12 and the reducing-agent powder 13 (including asource of a carbon dioxide gas added and mixed) are charged into thereaction container 11 with an apparatus for charging materials 14 shownin, for example, FIGS. 3A and 3B to provide layers in the form ofhelices. The reaction container 11 preferably includes, for example, acylindrical reaction container, called a sagger, composed of siliconcarbide (SiC). The shape of the reaction container 11 is not limited,but it is believed that a cylindrical shape is the most advantageous forthe reaction container 11. Furthermore, the dimensions of the reactioncontainer are not limited. However, in cylindrical shape, the reactioncontainer preferably has an inner diameter of about 200 to about 800 mmand has a height of about 100 to about 2000 mm. An amount of the mass ofsponge iron manufactured per container is preferably at least about 10.kg, from the viewpoint of productivity, more preferably at least about50 kg, and most preferably at least about 100 kg.

The reaction container 11 into which the iron oxide powder 12, thereducing-agent powder 13, and, if necessary, limestone and the like ischarged is placed on, for example, a truck and is disposed at a furnacesuch as a tunnel furnace. Then, the reduction is performed by heatingthe materials charged into the container for a predetermined time withthe container. This reduction is called a “rough reduction”. The puritytarget (metallic iron content in sponge iron after the reduction) isdetermined depending on an application of the reduced iron powder and isat least about 90 percent by mass, and in an application that requireshigh purity, at least about 97 percent by mass. The purity target has noupper limit. However, the purity achieved within the allowable costs isabout 99.5 percent by mass at the maximum under the present conditions.

Unsatisfactory heating temperature for the reduction leads to theinsufficient reduction of iron oxide, thus decreasing the purity of theresulting sponge iron. The lower limit of the heating temperature ispreferably about 1000° C. On the other hand, excessively high heatingtemperature excessively sinters sponge iron simultaneously with thereduction to harden. As a result, electric power consumption can beincreased when roughly pulverizing or manufacturing costs can beincreased due to wear and tear on a pulverizing tool. The upper limit ofthe heating temperature is preferably 1300° C. Consequently, the heatingtemperature is preferably in the range of 1000° C. to 1300° C.

When a tunnel furnace is used, the reaction container 11 (and iron oxidein the container) that is placed on a truck and moved in the furnacepasses through a preheating zone, where the temperature is graduallyincreased, over a period of about 24 hours (preferably between 20 and 28hours) and is retained in a firing zone at about 1000° C. to about 1300°C. for about 60 hours (preferably at least 36 hours and more preferablyat least 56 hours; and preferably up to 72 hours and more preferably upto 64 hours). After passing through a cooling zone where the temperatureis gradually reduced (preferably over a period of 20 to 28 hours), thereduction treatment is completed. The inlet temperature of thepreheating zone and the outlet temperature of the cooling zone arepreferably about 200° C. (about 20° C. to about 400° C.), while theoutlet temperature of the preheating zone and the inlet temperature ofthe cooling zone are preferably about 900° C. (about between (thetemperature of the firing zone)−450° C. and (the temperature of thefiring zone)−50° C.), from the viewpoint of, for example, the protectionof the reaction container (refractory).

Iron oxide is reduced with a reducing agent to produce a mass of spongeiron by such a thermal reduction reaction. The resulting sponge iron isnecessarily a mass that is in the form of helix. FIG. 7 shows an exampleof an appearance (the top end and the bottom end are omitted) of spongeiron produced by a method of the present invention.

A larger height (the axial direction) of the resulting mass of spongeiron is preferable. However, in view of the limitation of the size of areaction container and the reduction of thermal efficiency resultingfrom the large size of a reaction container when heightening a reactioncontainer, a mass of sponge iron preferably has a height of about 2000mm or less.

A method of the present invention can provide high-purity sponge ironhaving a purity of 97 percent by mass or more. When the purity is atleast 97 percent by mass, the product characteristics of sinteredcomponents such as mechanical components and magnetic materials or ofreduction iron powder that is used in the form of powder as-is areadvantageously guaranteed. However, a method of the present inventionhas the advantage other than purity and thus is not limited to a methodfor manufacturing sponge iron having a purity of at least 97 percent bymass or having high purity. That is, a method of the present inventioncan be generally applied to a usually rough reduction providing spongeiron having a purity of at least 90 percent by mass. Components otherthan produced metallic iron generally contains iron oxide and impuritiessuch as silicon (Si), manganese (Mn), phosphorous (P), and sulfur (S),the impurities being in an amount of up to one percent by mass in total.

After heating for the rough reduction, produced sponge iron is separatedfrom a reducing agent and is removed from the reaction container 11. Theresulting sponge iron removed from the reaction container 11 is roughlypulverized for a finishing reduction into powder generally having aparticle size of about 150 μm or less, thus resulting in roughly reducedparticles. Next, the roughly reduced particles are disposed in afinish-reducing furnace with a reducing atmosphere and are subjected tofinishing reduction, and are then further pulverized, thus resulting inreduced iron powder.

[Ratio of Iron Oxide to Reducing Agent]

In charging materials into a reaction container, the ratio of the amountof iron oxide to the amount of a reducing agent (solid reducing agent)when the above-described interwound helical charging is performed, inparticular, the ratio of carbon content in a reducing agent required foroxygen content in iron oxide has already been described above accordingto equation (2). That is, the ratio is determined based on the reductionreaction in which one carbon atom in a reducing agent reacts with oneoxygen atom in iron oxide ((the carbon content)/(the oxygen content)=1.0(molar ratio)). However, a reducing agent needs to generally have acarbon content larger than the oxygen content in iron oxide. In a knownmethod, the carbon content in a reducing agent is excessively charged,that is, is 2.0 to 2.5 times the oxygen content in iron oxide ((carboncontent)/(oxygen content)=2.0 to 2.5 (molar ratio)) because of theabove-described reasons. In this case, a reduction ratio (the puritytarget of sponge iron) is at least 90 percent by mass and preferably atleast 97 percent by mass in metallic iron.

The inventors have investigated the relationship between (the carboncontent)/(the oxygen content) (molar ratio) and the time required forthe reduction in a method for interwound helical charging by thefollowing experiments.

As shown in FIG. 8, to simplify the experiments, a method for chargingin the form of not helices but horizontally alternating charging wasemployed. That is, the iron oxide powder 12 and the reducing-agentpowder 13 are alternately charged to provide alternating layers that aresubstantially horizontal. The horizontally alternating charging producessponge iron in the form of a plurality of disks by reduction, thuscausing the operation to be complicated. Therefore, the interwoundhelical charging has an advantage over the horizontally alternatingcharging in actual use. However, from the viewpoint of the relationshipbetween (the carbon content)/(the oxygen content) (molar ratio) and theprogress of the reduction reaction, the horizontally alternatingcharging is equivalent to the interwound helical charging. Hereinafter,the horizontally alternating charging and the interwound helicalcharging are generically referred to as “alternating charging”.

A reaction container used for the experiments has an inner diameter of370 mm, and materials are charged such that the charged materials have aheight of 1400 mm. Iron oxide powder and reducing-agent powder used werethe same materials used in Example 1 described below. Reductiontreatment is performed at a maximum temperature of 1150 ° C. A reductiontime represents a retention time at this maximum temperature.

FIG. 9 is a graph showing the relationship between the ratio of thecarbon content to the oxygen content (in molar ratio) and the reductiontime required for producing metallic iron having a purity of 97 percentby mass with reference to various thicknesses of iron oxide layers in amethod for horizontally alternating charging. The molar ratio is theratio of the carbon content in and all reducing agent to the oxygencontent in all iron oxide.

As shown in FIG. 9, the filled circle (conventional example ●)represents an example of the result of the same reduction treatment witha general process for charging in a cylindrical form (shown in FIG. 1).In this general process, each of the iron oxide layers had a thicknessof 55 mm, (the carbon content)/(the oxygen content) (molar ratio) was2.2. The reduction time required was as much as 53 hours.

Iron oxide layers having thicknesses of 15 mm (Experimental Example 4:cross (x)), 20 mm (Experimental Example 3: triangle), 30 mm(Experimental Example 2: square (▪)), and 50 mm (Experimental Example 1:rhombus (♦)) provided by horizontally alternating charging (as shown inFIG. 8) were reduced. As a result, a smaller thickness of the iron oxidelayer led to the shortening of the reduction time. In the case of alayer having a thickness of at least 20 mm, when the molar ratio was 1.2or more, the reduction time was substantially constant. It was foundthat the molar ratio did not need to be 2.0 or more.

When the molar ratio is less than 1.2, it tends to prolong the reductiontime. However, alternating from a process for charging in a cylindricalform to a method of alternating charging and the effect resulting fromthe reduction of the thickness of layers predominantly counteract thetendency of the prolongation of the reduction time. That is, more ironoxide can be charged by a method for helical charging. For example, inthis example, a method for charging iron oxide having a thickness of 30mm in an interwound helical form can charge substantially the sameamount of iron oxide charged by a general process for charging in acylindrical form. Therefore, in the experimental range where the molarratio is 1.1 or more, the effect of the present invention issufficiently obtained. In addition, when the molar ratio is 1.15 ormore, the effect of the present invention is more sufficiently obtainedbecause of a small degree of prolongation of the reduction time. As amatter of course, when the molar ratio is 1.2 or more, the reductiontime is further shortened.

When the thickness of each iron oxide layer was 15 mm, the reductiontime was substantially constant at a molar ratio of 1.6 or more.Resulting from repeated experiments under the different conditions, itwas also found that, in an oxygen iron layer having a thickness of lessthan 20 mm, the following relationship holds:(molar ratio)×(thickness of iron oxide layer (mm))=2.3 to 2.5  equation(3)

When the thickness of each iron oxide layer is less than 20 mm, bycharging so as to satisfy equation (3), the determination of thethickness of each iron oxide layer necessarily leads to the reductiontime, thus resulting in a stable operation and a stable quality ofsponge iron produced. However, this relationship can be due to thedifficulty in stably controlling thinner thickness of eachreducing-agent layer rather than an essential relationship based on therate of reaction. Hence, it is expected that the above-describedlimitation is relaxed as an improvement of a technique in controllingthe thickness of layers.

From the viewpoint of the yield of a reducing agent, (the carboncontent)/(the oxygen content) (molar ratio) preferably is not increased.When the molar ratio is less than 2.0, a method of the present inventionhas an advantage compared with a general process for charging in acylindrical form. The molar ratio is preferably 1.8 or less.

As shown in FIG. 6, when a reducing-agent layer is provided at theperipheral portion in a container or a central axial portion of thecontainer, the inventors thought that it was necessary to study whetherthe regulation of (the carbon content)/(the oxygen content) molar ratio)in the entire container alone was adequate as a measure in designing theratio of the thicknesses of a reducing-agent layer and a iron oxidelayer.

To determine the amount required of a reducing agent at the portion ofthe deposited layers of materials (an intermediated portion in the formof a cylinder) in a reaction container, the inventors conductedexperiments whether any tendency was observed in reduction behavior withthe ratio of the thicknesses of a reducing-agent layer and an iron oxidelayer.

The experiment and the result will be described below.

That is, the molar ratio of the carbon content in a reducing agent tothe oxygen content in iron oxide charged in a reaction container wasfixed at 1.2. An experiment for changing the carbon content in areducing agent to the oxygen content in iron oxide in a portion wherethe iron oxide and the reducing agent were disposed in the form ofalternating layers excluding the reducing agent provided at a portionnear the wall (peripheral portion) of the reaction container and at acentral portion along the axial direction was performed.

This experiment was performed with a method for horizontal charging asin the above-described experiment. FIG. 10 shows a schematicalcross-sectional view of the state of charged materials. Thereducing-agent layers provided at the top region and the bottom regionof the intermediate portion are also included in the intermediateportion. Materials and the experimental conditions were the same as theabove-described experiment.

FIG. 11 is a graph showing the relationship between (the carboncontent)/(the oxygen content) (molar ratio) and the reduction time withreference to various thicknesses of iron oxide layers. The filledcircles (●) in the graph are the results from when the process forhorizontally alternating charging as shown in FIG. 8, the reducing-agentlayers being not provided at the peripheral portion and at the centralaxial portion in the process.

As shown in FIG. 11, iron oxide layers that were defined as four levels,that is, the iron oxide layers having thicknesses of 60 mm (ExperimentalExample 11: rhombus (♦)), 50 mm (Experimental Example 12: square (▪)),30 mm (Experimental Example 13: triangle), and 20 mm (ExperimentalExample 14: cross (x)) were reduced. As a result, a smaller thickness ofthe iron oxide layer led to the shortening of the reduction time. It wasfound that when (the carbon content)/(the oxygen content) (molar ratio)was 0.5 or more, the reduction time was substantially constant, whilewhen (the carbon content)/(oxygen content) (molar ratio) was less than0.5 the reduction time was prolonged.

Consequently, to maximally take advantage of the effect obtained when(the carbon content)/(the oxygen content) (molar ratio) is 1.2 or morein the entire container, it was found that (the carbon content)/(theoxygen content) (molar ratio) was preferably at least 0.5 at thecylindrical intermediate portion, and the cylindrical intermediateportion being the charged portion being in the form of helices(interwound helices).

To verify these results, another experiment was performed as follows:The molar ratio of the carbon content in a reducing agent to the oxygencontent in iron oxide at the cylindrical intermediate portion was fixedat 0.8. The amounts of a reducing agent charged into the peripheralportion and the central axial portion of the reaction container werevaried. FIG. 12 shows the results and is a graph showing the change inreduction time to (all carbon content)/(all oxygen content) (molarratio) in the entire reaction container. Each of the same symbols usedin FIGS. 11 and 12 represents the same thickness.

As shown in FIG. 12, it was found that when the molar ratio of (thecarbon content)/(the oxygen content) in the entire reaction container is1.2 or more, the reduction time is substantially constant, while whenthe molar ratio is less than 1.2, the reduction time is prolonged.

However, as described above, even if the molar ratio is less than 1.2,the effect of the present invention can be obtained if the molar ratiois 1.1 or more and preferably 1.15 or more.

In summary, in charging iron oxide and a reducing agent into thereaction container 11 in the form of alternating layers (such asinterwound helical charging) according to the present invention, theratio of the reducing agent to the iron oxide charged in the entirereaction container 11 that includes the peripheral portion, thecylindrical intermediate portion, and the central axial portion of thereaction container 11 is determined such that the molar ratio of thecarbon content in the reducing agent to the oxygen content in the ironoxide is preferably at least 1.1, more preferably at least 1.15, andmost preferably at least 1.2.

The thickness ratio of a reducing agent layer to an iron oxide layer atthe cylindrical intermediate portion that is charged in the form of(interwound) helices is preferably determined such that the molar ratioof the carbon content in the reducing agent to the oxygen content in theiron oxide is at least 0.5.

EXAMPLES Example 1

In this example, experimental levels as shown in Table 1 were defined.Iron oxide and a reducing agent were charged into the reaction container11 composed of silicon carbide (SiC) according to the experimentallevels and then roughly reducing treatment was performed to producesponge iron. Each of levels A to C and H is an example of a process forcharging in a cylindrical form as shown in FIG. 1. Each of levels D to Fis an example of a method for interwound helical charging as shown inFIG. 6. Level G is an example of a method for horizontally alternatingcharging.

In Table 1, 20% of the increment of the charge of Levels A and Drepresents that the sum of the thicknesses of layers composed of millscale in the reaction container 11 was increased by 20%; 40% of theincrement of the charge of Levels B and E represents that the sum of thethicknesses of layers composed of mill scale in the reaction container11 was increased by 40%; and 60% of the increment of the charge ofLevels C and F represents that the sum of the thicknesses of layerscomposed of mill scale in the reaction container 11 was increased by60%. The conditions are described in detail in Table 2. Under theseconditions, each Level was studied to determine a method for charging, asuitable thickness of a layer, and purity.

In this experiment, mill scale generated in a hot rolling step wasdried, pulverized, and screened. The mill scale powder used included 40percent by mass of particles that can pass through 60 μm mesh (it wasanalyzed that the mill scale powder had an average particle size withina range of 0.05 to 10 mm). A mixture of limestone powder andcarbonaceous powder was used as a reducing agent that was an auxiliarymaterial. The carbonaceous powder was produced by mixing coke andanthracite at the coke to the anthracite ratio of about 7:3. The cokeused had an average particle size of 85 μm and the anthracite used hadan average size of 2.4 mm. The content of the limestone powder having anaverage particle size of 80 μm in the entire reducing agent powder wasabout 14 percent by mass.

A reaction container was a cylindrical container having an innerdiameter of 400 mm. For charging in a cylindrical form, iron oxide wascharged so as to form a cylindrical shape having an outer diameter of320 mm, having a thickness of each value represented in Table 2, andhaving a height of about 1500 mm (the axial direction). For helicalcharging, a reducing-agent layer was provided with a diameter of about80 mm at the central axial portion and with a thickness of about 15 mmat the peripheral portion. Interwound charging was performed at theremaining cylindrical intermediate portion according to Table 2. Theresulting charged cylindrical intermediate portion had a height of about1500 mm. The molar ratio of the carbon content to the oxygen content inthe entire container and at the cylindrical intermediate portion that isin the form of a cylinder were at least 1.2 and at least 0.5,respectively. TABLE 1 Method for Increment of Charging Step of Levelcharging charge time charging A Charging in 20% 45 min Continuouscylindrical form B Charging in 40% 45 min Continuous cylindrical form CCharging in 60% 45 min Continuous cylindrical form D Charging in 20% 35min Continuous interwound helical form E Charging in 40% 35 minContinuous interwound helical form F Charging in 60% 35 min Continuousinterwound helical form G Horizontally  0% 90 min Discontinuousalternating charging H Charging in  0% 45 min Continuous cylindricalform

TABLE 2 Method for Increment of charging productivity 0% 20% 40% 60%Charging in Thickness of 20 mm 40 mm 60 mm 80 mm interwound iron oxidehelical form layer (vertical direction) Thickness of 30 mm 43 mm 47 mm45 mm reducing-agent layer (vertical direction) Charging in Thickness of57.5 mm 73.5 mm 93.5 mm 122 mm cylindrical iron oxide form layer (radialdirection)

Horizontally alternating charging was performed in order to verify theefficiency of charging. That is, the charging was performed by thefollowing procedure: An apparatus for charging materials used was thesame as for interwound helical charging. The rotatable charging cylinderwas rotated while charging any one of iron oxide powder orreducing-agent powder and was moved upward. Next, another powder wascharged by the same way. This procedure was repeated. As shown in Table1, the horizontally alternating charging cannot continuously charge andrequired a longer charging time than those of the charging in acylindrical form and the interwound helical charging. The interwoundhelical charging had the shortest charging time.

Reaction containers 11 each being charged with materials according tothe corresponding Level were placed on one truck and disposed in atunnel furnace. The truck passed through a preheating zone over a periodof about one day 200° C. to 900° C.) and a firing zone 1150° C.) over aperiod of about three days and then a cooling zone over 200° C. to 900°C.) a period of about one day. The truck was removed from the tunnelfurnace, and sponge iron was removed from the container. The purity ofthe resulting sponge iron was measured. All resulting sponge ironweighed 200 kg or more.

The purity of sponge iron was given by converting the metallic ironcontent in a chemical composition determined by a method for analyzingoxygen. FIG. 13 shows the results.

As shown in FIG. 13, in the case of the interwound helical charging(hatching patterned bars), iron oxide was excellently reduced to producehigh-purity sponge iron, which had a purity of above 97 percent or above98 percent by mass, when an iron oxide layer had a thickness of up to 60mm, i.e., the increment of productivity is up to 40%. It was found thatproductivity can be adjusted by controlling the thickness of the layerup to 40% of the increment of the charge compared with a known process.In the case of the charging in a cylindrical form, when the increment ofthe charge was 20%, the thickness of the layer was 75 mm and the puritywas 95.65 percent by mass; hence, productivity cannot be improvedcompared with the interwound helical charging.

Example 2

Sponge iron was manufactured according to Inventive Examples 1 to 5 andConventional Example 1. A method for charging as shown in FIG. 3A wassubstantially employed. (The carbon content)/(the oxygen content) (molarratio) was 1.2 or more.

Inventive Example 1

In this Inventive Example, an iron oxide layer having a thickness of 50mm and a reducing-agent layer having a thickness of 50 mm were chargedin the form of interwound helices. A cylindrical reaction container wasused with a height of 1.8 m and with an inner diameter of 40 cm. Amixture of coke powder having a particle size of up to 1 mm and 16percent by mass of limestone having an average particle size of about 95μm was used as the reducing-agent powder. Pulverized mill scale having aparticle size of up to 0.1 mm (after pulverizing, the mill scale wasscreened. The resulting mill scale included 40 percent by mass ofparticles that can pass through 60 μm mesh) was used as the iron oxidepowder. Both mill scale powder and coke powder had an average particlesize within a range of 0.05 to 10 mm.

An apparatus for charging materials as shown in FIG. 4A was used. Thecharging was performed as follows: The height of the opening of theoutlet for iron oxide powder 15 was adjusted to 50 mm. The height of theopening of the outlet for reducing-agent powder 16 was also adjusted to50 mm. The rotatable charging cylinder 14 b was operated at a rotatingspeed of 4 rpm and at a rising speed of 400 mm/min.

As a result of the charging, charged interwound helices each having 17turns were obtained, wherein the iron oxide layer had a thickness of 50mm and the layer of solid (powder) reducing agent had a thickness of 50mm. The charged iron oxide weighed 339 kg.

Inventive Example 2

In this Inventive Example, an iron oxide layer having a thickness of 35mm and a reducing-agent layer having a thickness of 65 mm were chargedin the form of interwound helices. Iron oxide and a solid reducing agentwere charged with the same reaction container, material powder, andapparatus for charging materials as Inventive Example 1. The chargingwas performed as follows: The height of the opening of the outlet foriron oxide powder 15 was adjusted to 35 mm. The height of the opening ofthe outlet for reducing-agent powder 16 was also adjusted to 65 mm. Therotatable charging cylinder 14 b was operated at a rotating speed of 4rpm and at a rising speed of 400 mm/min.

As a result of the charging, charged interwound helices each having 17turns were obtained, wherein the iron oxide layer had a thickness of 35mm and the layer of solid reducing agent had a thickness of 65 mm. Thecharged iron oxide weighed 237 kg.

Inventive Example 3

In this Inventive Example, an iron oxide layer having a thickness of 60mm and a reducing-agent layer having a thickness of 40 mm were chargedin the form of interwound helices. Iron oxide and a reducing agent werecharged with the same reaction container, material powder, and apparatusfor charging materials as Inventive Example 1. The charging wasperformed as follows: The height of the opening of the outlet for ironoxide powder 15 was adjusted to 60 mm. The height of the opening of theoutlet for reducing-agent powder 16 was also adjusted to 40 mm. Therotatable charging cylinder 14 b was operated at a rotating speed of 4rpm and at a rising speed of 400 mm/min.

As a result of the charging, charged interwound helices each having 17turns were obtained, wherein the iron oxide layer had a thickness of 60mm and the layer of solid reducing agent had a thickness of 50 mm. Thecharged iron oxide weighed 406 kg.

Inventive Example 4

In this Inventive Example, an iron oxide layer having a thickness of 25mm and a reducing-agent layer having a thickness of 25 mm were chargedin the form of interwound helices. Iron oxide and a reducing agent werecharged with the same reaction container, material powder, and apparatusfor charging materials as Inventive Example 1. The charging wasperformed as follows: The height of the opening of the outlet for ironoxide powder 15 was adjusted to 25 mm. The height of the opening of theoutlet for reducing-agent powder 16 was also adjusted to 25 mm. Therotatable charging cylinder 14 b was operated at a rotating speed of 4rpm and at a rising speed of 200 mm/min.

As a result of the charging, charged interwound helices each having 34turns were obtained, wherein the iron oxide layer had a thickness of 25mm and the layer of solid reducing agent had a thickness of 25 mm. Thecharged iron oxide weighed 339 kg.

Inventive Example 5

In this Inventive Example, an iron oxide layer having a thickness of57.5 mm and a reducing-agent layer having a thickness of 50 mm werecharged. Iron oxide and a reducing agent were charged with the samereaction container, material powder, and apparatus for chargingmaterials as Inventive Example 1. The charging was performed as follows:The height of the opening of the outlet for iron oxide powder 15 wasadjusted to 57.5 mm. The height of the opening of the outlet forreducing-agent powder 16 was also adjusted to 50 mm. The rotatablecharging cylinder 14 b was operated at a rotating speed of 4 rpm and ata rising speed of 430 mm/min.

As a result of the charging, charged interwound helices each having 16turns were obtained, wherein the iron oxide layer had a thickness of57.5 mm and the layer of solid reducing agent had a thickness of 50 mm.The charged iron oxide weighed 366 kg.

Conventional Example 1

In this example, charging in a cylindrical form was performed accordingto a known process as shown in FIG. 1. The same reaction container asExample 1 was used. Iron oxide powder was charged in the form of acylinder with a thickness of 57.5 mm and with an outer diameter of 310mm. A reducing-agent powder was charged around the iron oxide layer(including the inside of the cylinder). The same reaction container andmaterial powder as Inventive Example 1 were used. (The carboncontent)/(the oxygen content) (molar ratio) in the container was about2.2.

Reduction treatment was performed with a tunnel furnace. A time requiredfor the reduction was investigated.

Table 3 summarizes the results.

The time required for the reduction represents a retention time at afiring zone (1150° C.) in order to produce sponge iron having a purityof 95% or more. Production per hour represents a value obtained bydividing the weight of charged iron oxide by the time required for thereduction.

As shown in Table 3, the method of the present invention significantlyimproves productivity compared with the conventional process. TABLE 3Inventive Inventive Inventive Inventive Inventive Conventional Example 1Example 2 Example 3 Example 4 Example 5 Example 1 Method for Charging ininterwound helical form Charging in charging cylindrical form Thicknessof iron 50 35 60 25 57.5 57.5 oxide layer (mm) Thickness of reducing- 5065 40 25 50 ≧50 agent layer (mm) Weight of iron 339 237 406 339 366 227oxide (kg) Reduction time (h) 62 52 78 40 74 75 Productivity 5.46 4.555.22 8.47 4.94 3.02 per hour (kg/h)

Example 3 Inventive Example 6

A layer composed of the reducing-agent powder 13 (coke powder) wasdeposited with a thickness of 30 mm at the bottom of the reactioncontainer 11 with the apparatus for charging materials as shown in FIG.4A. Iron oxide powder 12 (mill scale) and reducing-agent powder 13 werecontinuously charged onto the bottom layer such that alternating layersof the iron oxide powder and the reducing-agent powder were formed andsuch that each of the layers was in the form of a helix, the iron oxidelayer having a thickness of 40 mm and the reducing-agent layer having athickness of 50 mm, while rotating the rotatable charging cylinder 14 bhaving the outlet for iron oxide powder 15 and the outlet forreducing-agent powder 16 and moving upward. Finally, the reducing-agentpowder 13 (coke powder) was charged at the top of the reaction container11. In this charging, the molar ratio of the carbon content in thereducing agent to the oxygen content in the iron oxide was 1.6. The sameconditions as EXAMPLE 2 were applied other than those above-described.

Comparative Example 1

Charging in the form of horizontal layers as shown in FIG. 8 wasperformed. In this example, charging was performed according to thefollowing procedure: In the apparatus for charging materials 14 as shownin FIG. 4A, the reducing-agent powder 3 (coke powder) was charged toform a layer having a thickness of 50 mm. Next, the iron oxide powder 12(mill scale) was charged on the reducing-agent layer to form a layerhaving a thickness of 40 mm. This charging procedure was repeated untilthe deposited layers reached the top end of the reaction container 11,providing that the reducing-agent powder 13 (coke powder) was charged atthe top end of the reaction container 11. The molar ratio of the carboncontent in the reducing agent to the oxygen content in the iron oxidewas 1.6.

Conventional Example 2

charging in a cylindrical form as shown in FIGS. 1A and 1B was performedas in Conventional Example 1 in EXAMPLE 2, but (the carbon content)/(theoxygen content) (molar ratio) was 2.5.

Next, the heat-resistant reaction container 11 containing materials wasplaced on a truck and passed through a tunnel furnace to heat and reduceiron oxide. The tunnel furnace having an entire length of 100 m wasused, and the atmospheric temperature was adjusted to 1150° C. at thecenter zone having a length of 40 m. Table 4 summarizes the results ofthe operations for manufacturing sponge iron having a purity of 97percent by mass under those conditions.

As is clear from Table 4, in this example of the present invention, thetruck speed was 1.3 m/h compared with 1.1 m/h of the ConventionalExample and was thus 18% faster than the Conventional Example. Theamount of mill scale charged was 256 kg per container compared with 220kg per container of the Conventional Example and was thus 16% greaterthan the Conventional Example. As a result, productivity was improved byas much as 38%. A quantity of heat per unit mass of iron oxide requiredfor heating can be reduced from 11,470 MJ/ton to 8,820 MJ/ton by as muchas about 30%. TABLE 4 Inventive Comparative Conventional Example 6Example 1 Example 2 Method for Charging in Horizontally Charging incharging interwound alternating cylindrical helical form charging formTruck speed (m/h) 1.3 1.3 1.1 Amount of mill 256 256 220 scale charged(kg/container) Retention time at 30.8 30.8 36.4 1150° C. (h) Heatconsumption 8820 8820 11470 rate (MJ/ton)

Example 4

Sponge iron was manufactured with an apparatus for charging materials asshown in FIG. 5. The same materials as EXAMPLE 2 were used. The cut-outsection 14 c had a semicircular shape (sector having a central angle ofabout 180°). A reaction container having an inner diameter of 400 mm anda height of 2,000 mm was used. A projection composed of a slag that wasformed by a reaction and adhered (maximum height was about 20 mm) waspurposely not removed and the rotatable charging cylinder was inserted.The main body of the rotatable charging cylinder had an outer diameterof 310 mm (77.5% of the inner diameter of the container). A virtualcircle at the horizontal cross-section of the cut-out section had adiameter of 360 mm (90% of the inner diameter of the container).

The rotatable charging cylinder can move to an opposite side when thefront end lightly came into contact with the projection or the reactioncontainer; hence, the rotatable charging cylinder was able to beinserted to the bottom of the reaction container with no problems, andthere is no problem when charging materials, that is, 260 kg of ironoxide powder was charged with no problems (a layer composed of ironoxide had a thickness of 50 mm, and a layer composed of a reducing agenthad a thickness of 30 mm).

After charging, the reduction was performed with no problems using atunnel furnace in the same way as EXAMPLE 2. As a result, a mass ofsponge iron having a helical shape was produced with a purity of 95percent by mass.

Example 5

Sponge iron was manufactured according to Inventive Examples 7 to 11,Comparative Example 2, and Conventional Example 3. A method for chargingas shown in FIG. 6 was performed.

In this EXAMPLE, iron oxide powder composed of mill scale and/or ironore was pulverized and screened in order to adjust the particle size,and was then used as the main material. Reducing-agent powder composedof at least any one of a simple substance or a mixture of coke powder,char, coal powder, charcoal powder, and the like was pulverized andscreened in order to adjust the particle size and was then used as amaterial. All materials had an average particle size of about 70 to 90μm.

An apparatus was used with a rotatable charging cylinder as shown inFIG. 14 was used. The operation was performed by the followingprocedure: The reducing-agent powder 13 was placed at the bottom of thereaction container 11, and the iron oxide powder 12 and thereducing-agent powder 13 were charged in the form of interwound heliceswhile rotating the rotatable charging cylinder 14 b of the apparatus forcharging materials 14 and simultaneously with moving upward at aconstant speed. The charging was performed up to the top end of thecontainer 1, provided that the top end of the reaction container 11 wascharged with the reducing-agent powder 13. To remove a product (spongeiron) from the container, to prevent sponge iron from adhering to thecontainer, and to enhance the efficiency of gas diffusion, the centralaxial portion and the peripheral portion near the wall were charged witha reducing agent.

Convention Example 3

In this example, a general process for charging was employed as shown inFIG. 1. An iron oxide layer having an outer diameter of 310 mm, an innerdiameter of 200 mm, and a length of 1,600 mm was formed in aheat-resistant reaction container 1 (inner diameter: 400 mm, length:1,800 mm) (provided that remaining portion was charged with a reducingagent). (The carbon content)/(the oxygen content) (molar ratio) was 2.2in the container. When the purity target was 97.0 percent by mass, thereduction time 1,150° C., hereinafter, all reductions were performed atthe same temperature.) was 53 hours.

Inventive Example 7

In this example, interwound helical charging was performed. An ironoxide layer had an outer diameter of 390 mm, an inner diameter of 60 mm,a thickness of 60 mm, and a helical shape. A reducing-agent layer had athickness of 45 mm and a helical shape. The outer diameter and the innerdiameter of the reducing-agent layer was the same as the iron oxidelayer. The iron oxide layer and the reducing-agent layer weresimultaneously formed. The molar ratio of (the carbon content in thereducing agent)/(the oxygen content in the iron oxide) was 0.8 in thecylindrical intermediate portion. (The carbon content)/(the oxygencontent) (molar ratio) was 1.2 in the all charged materials. As aresult, the amount of materials charged was increased by 35% comparedwith Conventional Example 3. However, the reduction time was as short as60 hours. The resulting sponge iron did not adhere to the inner face ofthe container and was readily removed from the container.

Inventive Example 8

In this example, interwound helical charging was performed. An ironoxide layer had an outer diameter of 365 mm, an inner diameter of 100mm, a thickness of 60 mm, and a helical shape. A reducing-agent layerhad a thickness of 28 mm and a helical shape. The outer diameter and theinner diameter of the reducing-agent layer was the same as the ironoxide layer. The iron oxide layer and the reducing-agent layer weresimultaneously formed. The molar ratio of (the carbon content in thereducing agent)/(the oxygen content in the iron oxide) was 0.5 in thecylindrical intermediate portion. The molar ratio of the carbon contentto the oxygen content was 1.2 in the all charged materials. As a result,the amount of materials charged was increased by 35% compared withConventional Example 3. However, the reduction time was 59 hours. Theresulting sponge iron did not adhere to the inner face of the containerand was readily removed from the container.

Inventive Example 9

In this example, interwound helical charging was performed. An ironoxide layer had an outer diameter of 350 mm, an inner diameter of 100mm, a thickness of 60 mm, and a helical shape. A reduced iron layer hada thickness of 17 mm and a helical shape. The outer diameter and theinner diameter of the reducing-agent layer was the same as the ironoxide layer. The iron oxide layer and the reducing-agent layer weresimultaneously formed. The molar ratio of (the carbon content in thereducing agent)/(the oxygen content in the iron oxide) was 0.3 in thecylindrical intermediate portion. The molar ratio of the carbon contentto the oxygen content was 1.2 in the all charged materials. As a result,the amount of materials charged was increased by 35% compared withConventional Example 1. However, the reduction time was 70 hours. Theresulting sponge iron did not adhere to the inner face of the containerand was readily removed from the container. However, the reduction timewas comparable with that in Conventional Example 3 even in view of theincrement.

Inventive Example 10

In this example, interwound helical charging was performed. An ironoxide layer had an outer diameter of 375 mm, an inner diameter of 100mm, a thickness of 60 mm, and a helical shape. A reducing-agent layerhad a thickness of 45 mm and a helical shape. The outer diameter and theinner diameter of the reducing-agent layer was the same as the ironoxide layer. The iron oxide layer and the reducing-agent layer weresimultaneously formed. The molar ratio of (the carbon content in thereducing agent)/(the oxygen content in the iron oxide) was 0.8 in thecylindrical intermediate portion. The molar ratio of the carbon contentto the oxygen content was 1.5 in the all charged materials. As a result,the amount of materials charged was increased by 20% compared withConventional Example 3. However, the reduction time was 59 hours. Theresulting sponge iron did not adhere to the inner face of the containerand was readily removed from the container. Inventive Example 7 with alow molar ratio of (the carbon content)/(the oxygen content) in thecontainer represented higher production efficiency per reduction timecompared with this example. However, this example represented excellentresults compared with the Conventional Example.

Inventive Example 11

In this example, interwound helical charging was performed. An ironoxide layer had an outer diameter of 395 mm, an inner diameter of 40 mm,a thickness of 60 mm, and a helical shape. A reducing-agent layer had athickness of 45 mm and a helical shape. The outer diameter and the innerdiameter of the reducing-agent layer was the same as the iron oxidelayer. The iron oxide layer and the reducing-agent layer weresimultaneously formed. The molar ratio of (the carbon content in thereducing agent)/(the oxygen content in the iron oxide) was 0.8 in thecylindrical intermediate portion. The molar ratio of the carbon contentto the oxygen content was 1.1 in the all charged materials. As a result,the amount of materials charged was increased by 40% compared withConventional Example 3. However, the reduction time was 78 hours. Theresulting sponge iron did not adhere to the inner face of the containerand was readily removed from the container. In this example, thereduction time was prolonged. The reduction time was comparable withthat in Conventional Example 3 even in view of the increment.

Table 5 summarizes the results. TABLE 5 Conventional Inventive InventiveInventive Inventive Inventive Example 3 Example 7 Example 8 Example 9Example 10 Example 11 Method for charging Charging in Charging ininterwound helical form cylindrical form Outer diameter of 310 390 365350 375 395 charge (mm) Inner diameter of 200 60 100 100 100 40 charge(mm) Thickness of iron 55 60 60 60 60 60 oxide layer (mm) Thickness ofreducing- ≧50 45 28 17 45 45 agent layer (mm) Molar ratio in 2.2 1.2 1.21.2 1.5 1.1 container Molar ratio at cylindrical — 0.8 0.5 0.3 0.8 0.8intermediate portion Weight of iron oxide 1 1.35 1.35 1.35 1.2 1.4(relative ratio) Reduction time (h) 53 60 59 70 59 78 Productivity perhour* 0.019 0.023 0.023 0.019 0.020 0.018*(Weight of iron oxide (relative ratio))/(Reduction time (h))

Industrial Applicability

As described above, according to the present invention, sponge iron canbe manufactured with high productivity and high quality (for example, ata purity of 97% or more) by employing the technique of interwoundhelical charging. Furthermore, since a structure formed by chargingmaterials into a reaction container can be changed to a desiredstructure easily and readily, quality, quantity, and a reduction timecan be easily adjusted; hence, production efficiency can besignificantly improved. As a result, high-purity sponge iron can bemanufactured at low cost.

1. A method for manufacturing sponge iron, comprising: a charging stepof charging iron oxide powder and reducing-agent powder into a reactioncontainer; and a reducing step of reducing the iron oxide powder in thereaction container to produce a mass of sponge iron by heating from theoutside of the reaction container, wherein, in the charging step, theiron oxide powder and the reducing-agent powder are charged such thatalternating layers of the iron oxide powder and the reducing-agentpowder are formed and such that each of the layers is in the form of ahelix.
 2. The method for manufacturing sponge iron according to claim 1,wherein, in the charging step, the iron oxide powder and thereducing-agent powder are charged such that layers composed of thereducing-agent powder are disposed on an inner side-surface of thereaction container (referred to as “peripheral portion”) and disposed ata central portion along the vertical central axis and such that thealternating layers that are in the form of helices are disposed at aportion (referred to as “intermediate portion”) other than the portionof the layers disposed on the inner side-surface and at the centralportion.
 3. The method for manufacturing sponge iron according to claim1, wherein the iron oxide powder comprises at least one selected fromthe group consisting of an iron ore, mill scale, and iron oxide powderrecovered from a waste pickling solution.
 4. The method formanufacturing sponge iron according to claim 1, wherein thereducing-agent powder comprises at least one selected from the groupconsisting of coke, char, and coal.
 5. The method for manufacturingsponge iron according to claim 1, wherein a source of a carbon dioxidegas is added to the reducing-agent powder.
 6. The method formanufacturing sponge iron according to claim 1, wherein the heatingtemperature is 1000° C. to 1300° C. in the reducing step.
 7. The methodfor manufacturing sponge iron according to claim 1, wherein, in thecharging step, the thicknesses of the layers of the iron oxide powderand the reducing-agent powder are variable when forming the layers thatare in the form of helices.
 8. The method for manufacturing sponge ironaccording to claim 1, wherein, in the charging step, the amounts of ironoxide powder and reducing-agent powder in the reaction container arecontrolled such that the molar ratio of the carbon content in thereducing-agent powder to the oxygen content in the iron oxide powder isat least 1.1.
 9. The method for manufacturing sponge iron according toclaim 2, wherein, in the charging step, the amounts of iron oxide powderand reducing-agent powder in the reaction container are controlled suchthat the molar ratio of the carbon content in the reducing-agent powderto the oxygen content in the iron oxide powder is at least 1.1.
 10. Themethod for manufacturing sponge iron according to claim 9, wherein, inthe charging step, the amounts of iron oxide powder and reducing-agentpowder in the intermediate portion are controlled such that the molarratio of the carbon content in the reducing-agent powder to the oxygencontent in the iron oxide powder is at least 0.5.
 11. A method formanufacturing reduced iron powder, comprising the steps of: pulverizingsponge iron manufactured by the method according to claim 1; reducingthe resulting pulverized iron; and repulverizing the resulting reducediron.
 12. Sponge iron having a helical shape.
 13. The sponge ironaccording to claim 12, wherein the sponge iron has a metallic ironcontent of at least 97 percent by mass.
 14. An apparatus for chargingmaterials used to manufacture sponge iron into a container, thematerials being iron oxide powder and reducing-agent powder, theapparatus comprising: a charger capable of rotating and verticallymoving in the container when the charger is disposed in the container;an outlet for the iron oxide powder and an-outlet for the reducing-agentpowder, these outlets being provided at the bottom of the charger andcapable of rotating together with the charger.
 15. The apparatus forcharging materials used to manufacture sponge iron into a containeraccording to claim 14, wherein the opening areas of the outlet for theiron oxide powder and the outlet for the reducing-agent powder can bevariable.
 16. The apparatus for charging materials used to manufacturesponge iron into a container according to claim 14, wherein the chargercomprises: a cylindrical main body having a diameter of up to 85% of theinside diameter of the container; and a lower end composed of part of acylinder, the horizontal section of the cylinder being a circle having adiameter of 90% to 95% of the inside diameter of the container, whereinthe horizontal section of the lower end has the shape of a sectorincluding the center of the circle and part of the circumference of thecircle, or has a shape including the sector.